Mutations of the T/t-complex of the mouse interfere with multiple cellular interactions during fertilization and development, including sperm-egg recognition, blastocyst formation and mesenchymal cell migration. In 1979, it was shown that these mutations selectively interfere with the expression of the cell surface enzyme, galactosyltransferase (GalTase). Consequently, during the past seven years we have been examining the function of this cell surface enzyme, GalTase, in those cell interactions that are controlled by T/t-complex alleles. Results show that surface GalTase participates in cell interactions during fertilization and development by recognizing and binding to its specific lactosaminoglycan (LAG) substrate on adjacent cell surfaces and in the extracellular matrix. In this way, GalTase functions as a cell surface "receptor" mediating, at least in part, sperm-egg binding and mesenchymal cell migration on basal lamina matrices. We have determined that surface GalTase also participates during embryonal carcinoma cell adhesions, as well as during cell adhesions in the preimplantation embryo. Thus, it appears that surface GalTase:LAG complexes may mediate, in part, those cellular interactions during fertilization and early development that are regulated by the T/t-complex alleles. It is now appropriate to examine the molecular control of GalTase expression. In this application, we propose to clone the embryonal carcinoma GalTase cDNA, study the transcriptional and translational heterogeneity of GalTase before and after embryonal carcinoma cell differentiation into endoderm, determine the effects of inhibiting GalTase expression on embryonal carcinoma cell differentiation, and determine GalTase gene number and heterogeneity in normal and T/t-mutant cells. These studies will offer new insights into the regulation of glycoconjugate heterogeneity in general, and the expression and function of surface GalTase, in particular. They will also lay the foundation for future studies in which we will perturb the expression of GalTase during development in transgenic mice, both positively and negatively, and examine the consequences on fertilization, blastocyst formation, and cell migration.